Nutrient consumption by diatoms in the dark subsurface layer of Funka Bay, Hokkaido, Japan

We conducted time-series observations in Funka Bay, Hokkaido, Japan, from 15 February to 14 April 2019. The diatom spring bloom peaked on 4 March and started declining on 15 March. Funka Bay winter water remained below 30-m depth, which was below the surface mixed-layer and dark-zone depths on both dates. At depths of 30–50 m, concentrations of NO3–, PO43–, and Si(OH)4 decreased by half between these dates even in darkness. Incubation experiments using the diatom Thalassiosira nordenskioeldii showed that this diatom could consume nutrients in darkness at substantial rates. We conclude that the nutrient reduction in the subsurface layer (30–50 m) could be explained by dark consumption by diatoms that had been growing in the surface waters and then sank to the subsurface layer. We believe that this is the first study to present observational evidence for the consumption of the main nutrients by diatoms in the dark subsurface layer during the spring bloom. Nutrient consumption in this layer might have a substantial influence on the primary production during and after the spring bloom.

Significant methane undersaturation during austral summer in the Ross Sea (Southern Ocean)

Dissolved methane (CH4) was measured at 9 stations along a transect at 75° S in the Ross Sea during austral summer in January 2020. CH4 undersaturation (mean: 82 ± 20 %) was found in the water column, with a mean air-sea CH4 flux density of −0.58 ± 0.48 μmol m−2 day−1, which suggests that the Ross Sea was a net sink for atmospheric CH4 during the austral summer. Simple box-model calculations revealed that the CH4 depletion should occur in the surface mixed layer because of CH4 oxidation and advection of CH4-poor waters. We propose that freshwater injection caused by sea-ice melting in summer dilutes CH4 concentrations within the surface layer and thus increases its potential for atmospheric CH4 uptake in the Ross Sea. Thus, we argue that both CH4 consumption and sea-ice melting are important drivers of CH4 undersaturation, which implies that the high-latitude area of the Southern Ocean is a sink for atmospheric CH4. We estimated that the Southern Ocean (> 65° S) takes up about 0.02 % of the global CH4 emissions and thus represents a minor sink for atmospheric CH4.

Nonseasonal Variations in Near-Inertial Kinetic Energy Observed Far Below the Surface Mixed Layer in the Southwestern East Sea (Japan Sea)

Near-inertial internal waves (NIWs) generated by surface wind forcing are intermittently enhanced below and within the surface mixed layer. The NIW kinetic energy below the surface mixed layer varies over intraseasonal, interannual, and decadal timescales; however, these variations remain unexplored, due to a lack of long-term, in situ observations. We present statistical results on the nonseasonal variability of the NIW kinetic energy 400 m below the surface mixed layer in the southwestern East Sea, using moored current measurements from 21 years. We used long time series of the near-inertial band (0.85–1.15 f) kinetic energy to define nine periods of relatively high (period high) and seven periods of relatively low (period low) NIW kinetic energy. The NIW kinetic energy average at period high was about 24 times higher than that at period low and those in specific years (2003, 2012–2013, 2016, and 2020) and decade (2010s) were significantly higher than those in other years and decade (2000s). Composite analysis revealed that negative relative vorticity and strong total strain significantly enhance NIW kinetic energy at 400 m. The relative vorticity was negative (total strain was positively enhanced) during seven (six) out of nine events of period high. NIW trapping in a region of negative relative vorticity and the wave capture process induce nonseasonal variations in NIW kinetic energy below the surface mixed layer. Our study reveals that, over intraseasonal, interannual, and decadal timescales, mesoscale flow fields significantly influence NIWs.

Subsurface iron accumulation and rapid aluminum removal in the Mediterranean following African dust deposition

Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (

Observed variability in the current field during summer monsoon experiments: Arabian Sea

The observed short term variability in the current field of the upper layers at selected locations in the Arabian Sea is examined utilising the available short (1-2 weeks) time series of moored currentmeter records obtained from former USSR stationary ship polygons during MONSOON-77 and MONEX-79 field experiments. Supplementary time series data sets on surface wind, sub-surface temperature and salinity were also made use of to explain the observed structure and variability of current field, in the upper 2OOm water column. The thermal regime in the central Arabian Sea showed cooling and deepening of the surface mixed layer with the onset and progress of the summer monsoon during MONSOON- 77 while the corresponding variability was marginal in the western and south-central Arabian Sea during pre-onset regime of MONEX-79, The Ekman balance appeared to be limited to the mixed layer, only during pre-onset regime of MONSOON-77 and was absent during pre-onset and onset regimes of MONEX-79 suggesting the importance of internal ocean dynamics influencing the current field. Most of the current records showed rich structure with superposed oscillations extending over the entire 200m water column. During progress regime of MONSOON-77 and at the equatorial station during pre-onset regime of MONEX-79. dramatic reduction in the current strength is noticed from mixed layer to thermocline due to differences in the eddy viscosity. During MONEX-79, a strong subsurface core of southerly flow ( -100 cm/s) was noticed at the equator (49°E) even before the onset of monsoon. The vector time series of current-meter records subjected to rotary spectral analysis showed inertial oscillations in the flow regime more prominently during MONSOON-77 as compared to MONEX-79. R.R. RAO. K. V. SANIL [email protected] and BASIL [email protected]

A Numerical Analysis on Coral Larval Networks across Reef Areas on the Northwest Coast of Okinawa Main Island, Japan

Coral bleaching has recently occurred extensively over the world’s oceans, primarily due to high water temperatures. Mesophotic corals that inhabit at depths of approximately 30–150 m are expected to survive during bleaching events and to reseed shallow water corals afterward. In particular, in Okinawa, Japan, mesophotic coral ecosystems (MCEs) have been reported to serve as a refuge to preserve genotypic diversities of bleaching-sensitive corals. Connectivity of larval populations between different habitats is a key element that determines the area to be conserved for desirable coral ecosystems. Coral larvae generally behave passively to the surrounding currents and are transported by the advective and dispersive effects of ambient ocean currents. Thus, numerical ocean circulation models enable us to quantify connectivity with detailed spatiotemporal network structures. Our aim in this study is to quantify the short-distance and vertical connectivity of coral larvae in reef areas on the northwest coast of Okinawa Main Island. For the reason that both short-distance and vertical larval transport are influenced by complex nearshore topography, a very high-resolution 3-D circulation model is required. Therefore, we developed a quadruple nested high-resolution synoptic ocean model at a lateral spatial resolution of 50 m, coupled with an offline 3-D Lagrangian particle-tracking model. After validation of the developed model, short-distance horizontal coral connectivity across reef areas on the northwest coast was successfully evaluated. Furthermore, a series of Lagrangian particle release experiments were conducted to identify the vertical coral migration and 3-D connectivity required for the preservation of MCEs. The model revealed that coral larvae released from the semi-enclosed areas tended to remain near the source area, whereas they were diffused and dispersed gradually with time. The mesophotic corals were dispersed vertically to the deeper zone below the mixed layer, while upward transport occurred to induce the mesophotic corals to emerge near the surface, under the influence of the surface mixed layer. The model results solidly indicated significant connectivity between MCEs and shallow coral ecosystems.

Unprecedented Outbreak of Harmful Algae in Pacific Coastal Waters off Southeast Hokkaido, Japan, during Late Summer 2021 after Record-Breaking Marine Heatwaves

Unprecedented large-scale harmful algae blooms (HABs) were reported in coastal waters off the south-eastern coast of Hokkaido, Japan, in mid-to-late September 2021, about a month after very intense and extensive marine heatwaves subsided. To understand the physical–biological processes associated with development of the HABs, we conducted analyses via a combination of realistic ocean circulation models, particle-tracking simulations, and satellite measurements. The satellite-derived chlorophyll concentrations (SCCs) and areal extent of the high SCCs associated with the HABs were the highest recorded since 1998. More specifically, the extent of SCCs exceeding 5 or 10 mg m−3 started to slowly increase after 20 August, when the marine heatwaves subsided, intermittently exceeded the climatological daily maximum after late August, and reached record-breaking extremes in mid-to-late September. About 70% of the SCCs that exceeded 10 mg m−3 occurred in places where water depths were <300 m, i.e., coastal shelf waters. The high SCCs were also tightly linked with low-salinity water (e.g., subarctic Oyashio and river-influenced waters). High-salinity subtropical water (e.g., Soya Warm Current water) appeared to suppress the occurrence of HABs. The expansion of the area of high SCCs seemed to be synchronized with the deepening of surface mixed layer depths in subarctic waters on the Pacific shelves. That deepening began around 10 August, when the marine heatwaves weakened abruptly. However, another mechanism was needed to explain the intensification of the SCCs in very nearshore waters off southeast Hokkaido. Particle-tracking simulations based on ocean circulation models identified three potential source areas of the HABs: the Pacific Ocean east of the Kamchatka Peninsula, the Sea of Japan, and the Sea of Okhotsk east of the Sakhalin Island. Different processes of HAB development were proposed because distance, time, and probability for transport of harmful algae from the potential source areas to the study region differed greatly between the three source areas.

The Early Paleogene Succession at Tora, New Zealand; Stratigraphy and Paeloclimate: A Critical North Island Eocene Temperature Record and a Crucial Linkage Between the Depositional Settings of the Central and Southern East Coast Basin

<p>This study has utilised the Mg/Ca paleothermometry method to provide a new, North Island reference of sea temperatures in the Southwest Pacific during a period of extreme global warming, referred to as the Early Eocene Climatic Optimum (EECO; ~53-50 Ma). This period of Earth’s history is of great interest as it represents the warmest climates of the Cenozoic. Importantly the climate dynamics of this period as simulated by computer models do not appear to match paleo-proxy data, specifically with regard to the latitudinal distribution of heat. Development of this paleoceanographic record involved detailed mapping of three sections in the Wairarapa region (41.506199 S, 175.517663 E) of New Zealand’s North Island. Three primary stratigraphic sections (Pukemuri, Awheaiti and Te Oroi Streams) were described and dated using foraminiferal and calcareous nannofossil biostratigraphy, with supplementary observations and measurements included from sections at Manurewa and Te Kaukau Points. These sediments are primarily siliciclastic sandstones and mudstones in composition, and sedimentary structures within these sections include turbidite sequences, channelisation and synsedimentary slumping, suggesting the EECO interval here is represented by sedimentation within a mid-bathyal submarine channel and fan environment. In contrast, the Early Paleocene Manurewa and Awhea Formations have previously been interpreted as a shallow, marginal marine environment which is at odds with benthic foraminiferal paleodepth indicators and trace fossil assemblages identified in this study. Selected genera of planktic foraminifera were extracted from the EECO interval and paleo-water temperatures determined from Mg/Ca values measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICPMS). This method was selected as it allows specific targeting of analysis sites, enabling avoidance of contaminated and altered parts of the test. This method also provides simultaneous measurements of other trace elements (Al, Si, Ti, Mn, Zn, Sr, Ba) that can be used as a guide to preservation state of the test (for example, Al, Ti and Si are considered indicators of detrital contamination levels). Four foraminifera genera were selected as suitable paleotemperature indicators of separate components of the water column. Morozovella spp. and Acarinina spp. were selected for surface mixed layer paleotemperature estimates, Subbotina spp. for thermocline temperature values, and Cibicides spp. for bottom water temperature determinations.SEM images, combined with trace element data were used to parse the resulting Mg/Ca data and only those that met strict quality criterion, including low detrital contamination and lack of visual evidence for recrystalisation were used for temperature reconstruction. Planktic Mg/Ca data were converted to temperature using the relationship (Mg/Ca = [Mg/Casw-t]/[Mg/Casw-0] × 0.38 0.09 × T) and benthic Mg/Ca temperatures calculated using (Mg/Ca = [Mg/Casw-t]/[Mg/Casw-0] × 0.87 0.109 × T), each assuming an early Eocene seawater Mg/Ca value of 4.1 mol/mol. Calibrated Mg/Ca results show peak sea surface temperatures of 29°C for Morozovella and Acarinina in the East Coast Basin during the Early Eocene, with bottom water temperatures of 17°C obtained from Cibicides. These data are consistent with the high sea surface temperatures reconstructed by previous workers in the nearby Canterbury Basin. The data from this new reference point support the idea that the EECO was characterised by a lower, possibly absent latitudinal temperature gradient in the midlatitude Southwest Pacific, than numerical models suggest, indicating a fundamental gap in the knowledge of climate dynamics in conditions much warmer than today.</p>